Chapter 2 Atoms, Molecules, and Ions

1 Atoms, molecules , and Ions Chapter 2 Atoms, molecules , and Ions Jim Geiger Cem 151 Atoms, molecules , and Ions Atomic Theory of Matter The theory of atoms: Original to the Greeks Leuccipus, Democritus and Lucretius (Aristotle thought they were nuts) He believed that one could divide up a piece of matter an infinite number of times, that is, one never came up with a piece of matter that could not be further divided. He suggested that everything in the world was made up of some combination of four elements: earth, fire, water, and air. The elements were acted upon by the two forces of gravity and levity. Gravity was the tendency for earth and water to sink, and levity the tendency for air and fire to rise.

2 John Dalton (1805-1808) Revived the idea and made it science by measuring the atomic weights of 21 elements. That s the key thing because then you can see how elements combine. Atoms, molecules , and Ions Dalton s Postulates Each element is composed of extremely small particles called atoms. Tiny balls make up the world Atoms, molecules , and Ions Dalton s Postulates All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements. O N Atoms, molecules , and Ions Dalton s Postulates Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.

3 (As far as Dalton knew, they couldn t be changed at all). O N O N Red O s stay Os and aqua N s stay N s. Atoms, molecules , and Ions Dalton s Postulates Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms. H N NH3 ammonia Chemistry happens when the balls rearrange Atoms, molecules , and Ions Law of Constant Composition Joseph Proust (1754 1826) Also known as the law of definite proportions. The elemental composition of a pure substance never varies. The relative amounts of each element in a compound doesn t vary. H N NH3 ammonia ammonia always has 3 H and 1 N.

4 Atoms, molecules , and Ions Law of Conservation of Mass The total mass of substances present at the end of a chemical process is the same as the mass of substances present before the process took place. 3H2 + N2 2NH3 ammonia The atoms on the left all appear on the right Atoms, molecules , and Ions The Electron Streams of negatively charged particles were found to emanate from cathode tubes. J. J. Thompson (1897). Maybe atoms weren t completely indivisible after all. Atoms, molecules , and Ions The Electron Thompson measured the charge/mass ratio of the electron to be 108 coulombs/g.

5 How? by manipulating the magnetic and electric fields and observing the change in the beam position on a fluorescent screen. Atoms, molecules , and Ions Millikan Oil Drop Experiment measured charge of electron Univ. Chicago (1909). How? Vary the electric field (E) until the drops stop. Vary the charge (q) on the drop with more X-rays. Get a multiple of Coulombs. The charge of 1 electron. Eq = mg You set E, measure mass of drop (m) & know g. Find q. Major result: q couldn t be any number. It was a multiple of Coulombs Atoms, molecules , and Ions Radioactivity: The spontaneous emission of radiation by an atom. First observed by Henri Becquerel.

6 (1903 Nobel Prize with Pierre and Marie Curie) Also studied by Marie and Pierre Curie. rays not particles particles of some sort. Stuff comes out of atoms, subatomic particles Atoms, molecules , and Ions Radioactivity Three types of radiation were discovered by Ernest Rutherford: (memorize the 3 types of particle) particles, attracted to negative electrode, so they have a positive charge, much more mass than negative stuff (turn out to be He nuclei) particles, attracted to positive electrode, so they have a negative charge, 1000s of times less massive (turn out to be electrons coming from nucleus). rays, no charge, no mass, like light.

7 Atoms, molecules , and Ions The Atom, circa 1900: Plum pudding model, put forward by Thompson. Positive sphere of matter with negative electrons imbedded in it. most of the volume = positive stuff because most of the mass is positive Expectation: density more or less uniform throughout. Atoms, molecules , and Ions Discovery of the Nucleus The Gold Foil Experiment Rutherford and Marsden shot particles at a thin sheet of gold foil and observed the pattern of scatter of the particles. Atoms, molecules , and Ions The Nuclear Atom Virtually all the particles went straight through Most of the atom essentially empty A few particles deflected,some straight back.

8 A very small part of the atom is very dense, impenetrable. The mass must be concentrated. The size of nucleus will be proportional to the # of highly scattered particles. Atoms, molecules , and Ions The Nuclear Atom Rutherford and Marsden postulated a very small, dense nucleus with the negative electrons around the outside of the atom. Most of the volume of the atom is empty space. Atoms, molecules , and Ions Other Subatomic Particles Protons were discovered by Rutherford in 1919. Have the positive charge in the atom. Neutrons were discovered by James Chadwick in 1932. Have mass like proton, but no charge. Why was it harder to discover them?

9 Atoms, molecules , and Ions Subatomic Particles Protons and electrons are the only particles that have a charge. Protons and neutrons have similar mass. The mass of an electron is so small we will often ignore it. Atoms, molecules , and Ions Symbols of Elements: depicting the subatomic particles Elements are symbolized by one or two letters. Atoms, molecules , and Ions Atomic Number All atoms of the same element have the same number of protons: The atomic number (Z) Atoms, molecules , and Ions Atomic Mass The mass of an atom in atomic mass units (amu) is approximately the total number of protons and neutrons in the atom. Atoms, molecules , and Ions Isotopes: Elements are defined by the number of protons.

10 Isotopes are atoms of the same element with different masses. Isotopes have different numbers of neutrons. 11 6 C 12 6 C 13 6 C 14 6 C # Neutrons 5 6 7 8 Atoms, molecules , and Ions Atomic Mass Atomic and molecular masses can be measured with great accuracy with a mass spectrometer. Heavier ion turns less in the magnetic field (more momentum, because of higher mass (mv)) (magnetic moments of ions similar). Aston, 1919. Atoms, molecules , and Ions Average Mass Because in the real world all the elements exist as mixtures of isotopes. And we measure many many atoms at a time Natural abundance Average mass is calculated from the isotopes of an element weighted by their relative abundances.